The present invention generally relates to an exposure method and apparatus, and more particularly to an exposure apparatus and method used to manufacture various types of devices including semiconductor chips, display devices, detecting devices, image-pickup devices, and a fine pattern used for the micromechanics.
The photolithography technology is used to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit, and a liquid crystal display device. The conventional photolithography technology employs a reduction projection exposure apparatus that uses a projection optical system to project a circuit pattern of a reticle or mask onto a wafer, etc. to transfer the circuit pattern.
The minimum critical dimension (“CD”) transferable by the projection exposure apparatus or a resolution is proportionate to a wavelength of exposure light, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Accordingly, use of the exposure light having a shorter wavelength advances with the recent demands for fine processing to the semiconductor devices, from an ultra-high pressure mercury lamp (g-line with a wavelength of approximately 436 nm) and i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). In addition, the high NA scheme of the projection optical system is also promoted and the projection optical system having an NA of 0.9 or greater is about to reduce to practice. Moreover, an immersion lithography is proposed which fills a space between the final lens surface of the projection optical system and the wafer with a medium having a refractive index of 1.0 or greater, such as the water, increasing an apparent NA of the projection optical system up to 1.0 or greater, and improving the resolution.
When the high NA scheme of the projection optical system proceeds, for example, when NA becomes greater than 0.9, a polarization state of the light incident upon the projection optical system tends to significantly affect the resolving power. Two perfect coherent lights interfere with each other when their electric-field vibration directions are parallel to each other, but do not interfere at all when their electric-field vibration directions are perpendicular to each other even if they are perfectly coherent.
FIG. 10 shows that two lights are incident from the air and form an image on a photosensitive material or resist PR applied on a wafer WF, where a z axis denotes the optical-axis direction, an x axis denotes a direction perpendicular to the z axis, and a y axis denotes a direction perpendicular to FIG. 10. The y-polarized light is a polarized light that has an electric-field vibration plane on the yz plane. The x polarized light is a polarized light that has an electric-field vibration plane on the xy plane. When the polarization states of both the two lights are the y-polarized light, their vibration planes are always parallel to each other, and perfectly interfere with each other. Therefore, a repetitive image with a contrast of 100% is formed on the image plane. On the other hand, when the polarization states of both the two lights are the x-polarized light, their vibration planes are not parallel to each other and do not completely interfere with each other. Therefore, a repetitive image's contrast is not 100% on the image plane. Here, contrast C is expressed by Equation 1 below, where Imax denotes a maximum value of the light intensity, and Imin denotes a minimum value of the light intensity:C=(Imax−Imin)/(Imax+Imin)  (1)
FIG. 11 shows that an angle between the two lights is greater than that in FIG. 10. Similar to the illustration in FIG. 10, when the polarization states of the two lights are the y-polarized light, their vibration planes are always parallel to each other. Thus, they interfere with each other, forming a repetitive image on the image plane with a contrast of 100%. On the other hand, when the polarization states of the two lights are the x-polarized light, the interference characteristic become worse due to the large angle between two lights than that in FIG. 10, lowering the contrast.
In FIGS. 10 and 11, the two lights are incident upon the resist applied to the wafer from the air. The air has a refractive index of 1, and the resist has a refractive index between 1.4 and 1.8. When the light is incident upon the resist from the air, the angle between the two lights decreases. In an immersion lithography that places on the resist RP a material IM having a refractive index greater than that of the air, the refractive index difference between the material IM and the resist PR is smaller than the refractive index difference between the air and the resist. Therefore, the refraction angle reduces at which the light is incident upon the material IM from the resist PR, and the angle between two lights increases.
As the NA of the projection optical system increase, control over the polarization state of the light incident upon the projection optical system becomes vital. Since an optimal polarization state differs according to reticle patterns, control or switch of the polarization state is needed according to the reticle patterns. The optical element and thin film, such as a antireflection coating and a reflection coating administered on the optical element, in the projection optical system have optical characteristics depending upon the polarization of the incident light. For example, the birefringent glass material has a different refractive index and a different aberration according to polarization directions of the incident light. In other words, the projection optical system causes a different aberration according to the polarization directions of the incident light.
Equations 2 and 3 below are met, where WAx denotes an aberration caused by the incident x-polarized light, WAy denotes an aberration caused by the incident y-polarized light, WARANDOM denotes an aberration caused by the incident non- or randomly polarized light, BWAx denotes an aberration depending upon only the x-polarized light, and BWAy denotes an aberration depending upon only the y-polarized light.WAx=WARANDOM+BWAx  (2)WAy=WARANDOM+BWAy  (3)
The aberration depending upon only the polarization has the same absolute value but inverse codes with respect to two orthogonal, polarized lights. Therefore, Equation 4 below is met:BWAx=−BWAy  (4)
Equations 5 and 6 are sufficient to reduce both the aberrations WAx and WAy:WARANDOM=0  (5)BWAx=−BWAy=0  (6)
Equation 6 requires the projection optical system to reduce the birefringence down to 0. For this purpose, for example, various exposure apparatuses are proposed, which adjust the birefringence by a crystal orientation and an angle incorporated to the projection optical system, or which adjust the projection optical system to cancel out a birefringence effect of the antireflection coating. In this case, the optical system includes plural elements made of a crystalline glass material with a birefringence caused by the crystal structure. These exposure apparatuses are disclosed in Japanese Patent Applications, Publication Nos. 2003-050349 and 2004-172328.
However, due to the manufacture errors, it is difficult to correct a birefringence amount of an entire projection optical system to completely 0 even if attempted in order to correct the polarization dependent aberration in assembly and adjustment of the projection optical system. The conceivable manufacture errors include, for example, a crystal orientation of the crystalline glass material and a rotational incorporation angle, an uneven thickness of the antireflection coating, and a stress distortion applied in mounting the projection optical system on the exposure apparatus. The influence of the polarization dependent aberration simply deteriorates the contrast in the conventional non-polarized light illumination, and the influence on the imaging performance is small and negligible. However, in the polarization controlled illumination, the aberration deteriorates the contrast, causes an image shift and defocus, etc., and changes these amounts whenever a polarization state is switched.